Depending of the brand of slideout topper; you can extend slideout about 6 to 8 inches and then pin the topper roller at the end caps. For an A&E, you should be able to rotate the tube, looking through the hole in the end cap find another hole in the axle and insert a nail, small screwdriver or carter pin. This will hold the tube spring from retracting. now remove the tube assembly from the bracket and fold it back onto the roof.
For a carefree you may have to drill a hole through the axle in alignment with a hole in the end caps.
Now with the roof only part of the way out you should be able to remove and replace the seal. Good luck and keep us posted.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7